This invention relates to an induction type AC machine. The machine can operate in the motoring or generating modes, similar to conventional AC motors of the induction or synchronous types. Single or polyphase machines are possible. For purposes of clarity, this description of the background art in relation to the invention, will generally keep in view the motoring modes of conventional 3 phase induction and synchronous types of motors.
In conventional 3 phase motors, a sinusoidal 3 phase AC input to the stator windings create a rotating magnetic field in the air gap. Ideally, this field has a sinusoidal spatial distribution, at any instant of time. It rotates at the speed, determined by the AC input frequency and the number of motor poles, called the synchronous speed.
In the induction motor, the rotor speed is less than the synchronous speed. The speed difference, known as the slip speed, is the relative speed necessary to induce AC currents of slip frequency in the rotor conductors, whereby the motor develops torque. The slip speed, and therefore the motor speed, change with variations in load torque. The present invention achieves operation at a synchronous speed, independent of load variations, while still utilizing the principle of induced rotor currents, by means of a technique to be described subsequently.
The conventional synchronous motor always runs at the synchronous speed, independent of load torque variations. This happens, because the rotor has fixed magnetic poles, which lock with the stator rotating field. The fixed poles of the rotor are implemented, either by the use of permanent magnets, or magnetizing DC current in a field circuit on the rotor. Permanent magnet synchronous machines have no easy way for control of the rotor flux. DC excited synchronous machine requires a DC source, external to the rotor circuit, to provide the field current. Slip rings and brushes are necessary, to feed the field current to the rotor, if the DC exciting current is to be drawn from stationary DC terminals. "Brushless excitation" systems are also in use, wherein a coupled exciter machine is used, which has stationary DC poles and rotating coils in which AC is induced. These are rectified and made available for feeding the field terminals of the synchronous machine without the need for slip rings and brushes. Every DC excited synchronous machine requires a DC source, which is external to the rotor circuit. In the present invention, the DC excitation is achieved by rectification of induced AC in the rotor circuit itself. The motor therefore rotates at a synchronous speed, as defined subsequently, independent of load variations.
Since a slip speed is necessary to induce voltages in the rotor, there is an apparent contradiction in the above statement, that the motor rotates at synchronous speed and still has induced currents in its rotor circuit, in the present invention. This contradiction is resolved by the fact that in this invention, the stator rotating field actually has two rotating components, which rotate at different speeds. Of these, the "main field", which ordinarily is the dominant component, has the same motor function in this invention, as the field in a conventional synchronous motor. The other component, which will be labelled here as the "auxiliary field", serves to induce currents in the rotor, even when the motor is rotating at the synchronous speed of the main field. The rotor also has a rectifier circuit, for converting the induced currents into DC. The detailed description of the invention, which follows, further explains this.